Nuclear fuels spent in nuclear facilities such as nuclear power plants are typically transported to reprocessing plants and then reprocessed. However, such spent nuclear fuels today are generated in an amount exceeding the reprocessing capacity. Thus, it is necessary to store spent nuclear fuels for a long period. In this case, spent nuclear fuels are cooled to a radioactivity level that makes the fuels suitable for transportation, and then placed in a cask as a nuclear shielding container and transported. Even at this stage, the spent nuclear fuels still emit radiation such as neutrons. Neutrons have high energy, and generate γ-rays to cause serious harm to the human body. For this reason, it is necessary to develop a neutron shielding material that can surely shield such neutrons.
Neutrons are known to be absorbed by boron. To make boron absorb neutrons, it is necessary to moderate the neutrons. Hydrogen is known to be most suitable as a substance for moderating neutrons. Accordingly, a neutron shielding material composition must contain a large amount of boron atoms and hydrogen atoms. Further, since spent nuclear fuels or the like as a neutron source generate decay heat, the fuels are exothermically subjected to a high temperature when sealed in a cask for transportation or storage. The highest temperature varies depending upon the types of spent nuclear fuels, however, it is said that the temperature of spent nuclear fuels for high burnup may reach about 200° C. in a cask. For this reason, a nuclear shielding material for use preferably endures under such high-temperature conditions for about 60 years as a reference storage period for spent nuclear fuels.
In this situation, use of a substance having a high hydrogen density, in particular, water as a shielding material has been proposed, and some of the proposals have been put into practice. However, water is difficult to be handled because it is a liquid, and is not suitable for a cask for transportation and storage, in particular. Moreover, when water is used, the internal temperature of a cask is 100° C. or more, and thus it is difficult to suppress boiling, disadvantageously.
For this reason, conventionally, a resin composition has been used as a material for a neutron shielding material, and an epoxy resin has been used in one of such resin compositions. Generally, there is a reciprocal relationship between hydrogen content and heat resistance in a resin composition. A resin composition having a high hydrogen content tends to have low heat resistance, and a resin composition having high heat resistance tends to have a low hydrogen content. An epoxy resin exhibits excellent heat resistance and curability, but tends to contain only a small amount of hydrogens indispensable for moderating neutrons. Therefore, an amine curing agent having a high hydrogen content has been conventionally used to compensate this drawback.
Japanese Patent Application Unexamined Publication No. 6-148388/1994 discloses a neutron shielding material composition containing a polyfunctional amine epoxy resin and having reduced viscosity for improving workability at ordinary temperature. The composition exhibits excellent pot life. Japanese Patent Application Unexamined Publication No. 9-176496/1997 discloses a neutron shielding material obtained by curing a composition comprising an acrylic resin, epoxy resin, silicone resin or the like with a polyamine curing agent. Sincethe amine has relatively high hydrogen content, the effect of moderating neutrons is improved. However, the amine moiety thereof is easily decomposed by heat. In addition, in order to compensate the lack of the hydrogen content in the epoxy component, a curing agent having a high hydrogen content but having rather lower heat resistance such as polyamine tends to be used, and the ratio of the curing agent component in the resin composition tends to be high. Accordingly, it has been demanded to develop a novel composition having sufficient durability necessary for storing a spent nuclear fuel for high burnup, rather than a conventional composition cured with an amine curing agent.
An object of the present invention is to provide a neutron shielding material composition which exhibits heat resistance more excellent than that of a conventional composition, and has ensured neutron shielding capability.